In some conventional systems, a radio frequency (RF) signal may be converted to an intermediate frequency (IF), and then from IF to a baseband signal, where the IF may be in the megahertz range. Generally, the RF signal may be mixed with a local oscillator signal that results in two sideband signals that are the sum of the frequencies of the two signals and the difference of the frequencies of the two signals. One of the two sideband signals may be chosen as an IF signal, and this IF signal may be the same for all received RF signals. Therefore, a radio that may receive a plurality of channels, such as an AM or FM radio, may tune to a particular station by changing the local oscillator signal frequency such that the IF remains constant. With a constant IF, most of the receive path may be common in the receiver.
Today, much of radio receiver development may be driven mostly by a great demand for mobile wireless communication devices, including handsets. With the ever-decreasing size of mobile handsets, capacities of smaller batteries may be an issue. As most of these handsets may use complementary metal-oxide semiconductor (CMOS) technology for analog to digital conversion, and for much of the processing of voice and data signals, a very important factor to consider may be that it may be advantageous for CMOS devices work at lower frequencies. This may be crucial since CMOS devices have power dissipation directly related to the speed at which the CMOS devices switch. The faster the frequencies, the faster the CMOS device switching speed, and therefore, the greater the amount of power consumed. Therefore, receivers may be designed to downconvert the high frequency RF, which may be in gigahertz range, to a lower frequency, preferably to a baseband frequency, as quickly as possible.
As a result, some receivers may utilize chips for digitally processing baseband signals, and may expect to receive the baseband signal, rather than an IF signal. To meet this need, some receiver architectures, for example, direct-conversion receivers, try to do away with IF by converting directly from RF to baseband, and therefore reduce power consumption by not processing IF signals, and cost by not having to deal with IF signals. However, with direct conversion, the reduced power consumption may be offset by strong drawbacks, such as DC-offset generation, 1/f noise, I/Q mismatch, excessive flicker noise in the baseband, and local oscillator (LO) leakage. In addition, a digital signal processor (DSP) may be required to perform complex digital processing of the digital signal for filtering and downconverting from the RF frequency.
If a measured signal-to-noise ratio (SNR) is less than a desired SNR, the DSP may need to perform, for example, distortion cancellation or other SNR reduction or mitigation function. Additionally, during direct conversion a majority of gain and filtering may be performed in a frequency band from DC to the signal bandwidth. In this process, a signal path's intrinsic DC offsets may be amplified. The dynamic range of the circuit may thereby be degraded. In addition, a DC offset may be created if the LO signal leaks to the RF front end and self-mixes. Since GSM systems may use modulation and system synchronization techniques that require DC information, it may not be feasible to simply remove the DC component. Rather, complex DSP processing may be required to reduce the DC offset while still keeping the information present in the DC signal.
Although direct conversion receivers may try to reduce parts count and try to reduce power consumption, additional complex digital signal processing, and its accompanying cost, may be required. Simply reducing component count as with direct conversion, because system design complexity may increase, as well as cost, may not be the desired route. Furthermore, the digital baseband signal may have to be converted to an analog signal for some baseband processors.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.